
& 



V' 



v<0 

C ^ ^ 




V v ..i^L*. «> 




^y a> ^ -J3K?/ ^ \ •• 




"*£. 
<£ 




% 



S -o 







1 »Z,*°* V V 5 s ,!oL' 




** x-* ' »:«;i:./^ *. v> *2ii^'^ 







"- V* 
















*' 



V 







»"%. 










A©* 



^,/':«^'^-.^ 






♦ ■$» 



* <*. v ^ 




vv 







9^ v 









<T 



^**_ 







4" "<o. *m' < 







cT 0_ 




/ V^V V^»*/ V^\/ % 






:* r> v 




1^ * v\ V ^ 












4 0° " 









iT- "of -' 




°o >}>• <f, Jj 




^.,., 






(5° 



A 




V .*^Lt*.CL 



%. "•"•"' $ 



4?^ V 



h\ U ^ SeMZf. ^ .«** .WlB?/fc\ ^ .,* 





























v< 



^ v 



,0* *o. 




sx 




.4 ».«^.._ ^ ^ v ,♦, 









c" *0. A* * 



G ^ *<> • » * A 










^^ 



* aV ^ 



O 









C»V^% 






y °^^^*/ \**r!*y V*^\°° \' 

v* v °'JH* \/ *m %<^ .-ate-, v* ^^ %.^ : 










4 O 




'oV 







7." ^o° 



J. **, 




J? "o 







C.\P 














: ^o* 



^°* 



«* 





c> 



*> 















ft O 'o „ i * A, >. */^ r «* I*.*" »"> ». * A -* 














"bv* : 



^ 








«&<? 

^ 










%t r %^-v v*^V %,%?%&. *> V 3 ^* V -o 




°<- '••• *p ... <*■ •••» 









^:»V* V \W-^ 






»'- «U < •* 






©©? a**"* "Wl 



6* "% *s?xy A 



W; A W? •* §8P A -m )i cap K « ;t< 



aA 

















- -t, 










:•- %.^.« ««•.>♦ ^',^&"\ >* ••«•. •«. .,*' ,-, 










°o_ ^™ 



'bV 




r ^ 



»' J. 



*0 



c ° "! c * o 






** v \ . l ^5p?> r /\ 







• ^' 



o 






w • 







,v 






r^ : JK': *** : MMi ^^ -*Jfe- \^ SMfa. \>/ :jSfe\ ^ 













i>; vie 



-%% 



^6* 



vV 



•bV 
* ^ 



















°*^-v .. v^\/ ^^^/ v*^> °^' 








, ^ 






I® 7 / ^ v % 



c o> . .^>;" o o >•' ,.i^% ^ co» ..j- % *o, ■ ■ .* v t . ^ . ^ 




•** A 




++4 












v v 



•>° ... v^'<^ %^ 5 / V^*> <*?frsj 














^ "%. *".^fe', ' A 4 °^ V 



\*&\S V-^^ v^V % 




*°^ 




Ac- 



v A' 



IC 9116 



Bureau of Mines Information Circular/1986 



Thick-Seam Mining in the Western 
United States— Geological 
Considerations 



By D. L. Boreck 




UNITED STATES DEPARTMENT OF THE INTERIOR 



^ CUn^ StDfe*. 3uLT€CUi_ of Mrr**) 



Information Circular 9TI6 



Thick-Seam Mining in the Western 
United States— Geological 
Considerations 



By D. L. Boreck 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 







<\0' 



Library of Congress Cataloging in Publication Data: 



Boreck, D. L. (Donna L.) 

Thick-seam mining in the western 
siderations. 


United States - 


-geological con- 


(Information 


circular ; 9116) 






Bibliography 


p. 17-18 






Supt. of Docs 


. no.: I 28.27: 9116. 






1. Coal mines and mining- West (U.S.) 2. Coal- 
Information circular (United States. Bureau of M 


Geology-West (U.S.) I. Title. II. Series: 
nes) ; 9116. 


TN295.U4 


[TN805.A5] 622 s [622'.334] 


86-600298 



CONTENTS 



Page 



Abstract 1 

Introduction 2 

Occurrence of thick and split coal seams 2 

Thick-seam mining methods 4 

Geology as a scientific tool 5 

Geologic factors affecting thick-seam development 5 

Cleats 5 

Joints and fractures 7 

Coal quality 7 

Petrography 8 

Roof 1 i thology 8 

Floor li thology 10 

Thickness and three-dimensional configuration of coal seam 10 

Continuity of the coal seam 11 

Rolls 12 

Spontaneous combustion 13 

Gas 13 

Water 15 

Conclusions 17 

References 17 

ILLUSTRATIONS 

1. Map of coal-bearing regions in Colorado, Utah, and Wyoming 2 

2 . Two thick-seam mining methods 4 

3. Generalized log of corehole showing depth and the degree of cleat develop- 

ment throughout the core 6 

4. Cross section through thick coal in the Danforth Hills Field, western 

Colorado 10 

5. Example of small fault 11 

6. Structure map drawn on top of coal seam 12 

7. Fire in strip operation in southern Colorado 14 

TABLES 

1. Summary of thick- and split-seam occurrences and development in Colorado, 

Utah, and Wyoming 3 

2. Known active or proposed underground mines working thick or split seams in 

Colorado, Utah, and Wyoming ; 3 

3. Importance of geologic factors for four thick-seam mining methods 16 





UNIT OF MEASURE 


ABBREVIATIONS 


USED 


IN THIS REPORT 


cm 


centimeter 


ra 2 /d 




square meter per day 


ft 


foot 


pet 




percent 


in 


inch 


St 




short ton 


m 


meter 


std ft 3 




standard cubic foot 



THICK-SEAM MINING IN THE WESTERN UNITED STATES- 
GEOLOGICAL CONSIDERATIONS 



By D. L. Boreck 1 



ABSTRACT 

Thick coal seams are common in the Western United States. Many seams 
are over 50 ft thick (some are over 200 ft thick) and are too deep to 
extract using surface methods. Currently, such deposits are developed 
using standard "eastern" mining methods which only extract a few feet of 
total seam thickness, often rendering the remaining coal unminable with 
current technology. Novel methods that increase thick-seam recovery 
have been developed and are currently being used in Europe. These 
methods — high-face single-pass and multislice longwall, longwall caving, 
and hydraulic mining — have great potential for use in the United States. 
Successful use of these methods is an objective of the Bureau of Mines. 
Their use requires, among other things, a full evaluation of geologic 
features common to thick coals. The objective of this report is to pre- 
sent and summarize those features that will affect the introduction of 
the methods into thick-seam mines in the Western United States. 

The geologic elements delineated are three-dimensional configuration 
of the seam, cleat development, joints and fractures, roof and floor 
lithology, and faulting. 



Geologist, Denver Research Center, Bureau of Mines, Denver, CO. 



INTRODUCTION 



Thick (which includes closely spaced) 
coal seams are common in the Western 
United States. The "thick coal reserve" 
base is made up of seams that are more 
than 15 ft thick. Also included in the 
reserve base are seams split by from less 
than 1 ft to greater than 30 ft . of in- 
terburden. As the seam height commonly 
mined in the United States is less than 
10 ft, 45 to 95 pet or more of a de- 
posit's original reserve may be lost un- 
less improved methods are introduced into 
western mines (_1_). 2 This western coal 
has high export potential and is a major 
source of royalty revenue. 

The Bureau of Mines is evaluating the 
feasibility of introducing European 
thick-seam mining methods into the United 



States. To predict the degree of success 
of such an introduction, geologic, geome- 
chanical, mining, economic, and safety 
factors that will affect the thick-seam 
methods are being examined. This report, 
which was derived from an analysis of 
published material, concentrates on de- 
fining the geologic aspects of thick-seam 
development. As one of several publica- 
tions scheduled to be completed by the 
Bureau on thick-seam mining technology, 
it is a first step toward the identifica- 
tion and development of appropriate min- 
ing systems for such deposits. 

The report addresses underground devel- 
opment of thick-seam deposits and does 
not cover surface mining methods or 
techniques. 



OCCURRENCE OF THICK AND SPLIT COAL SEAMS 



Because of differing mining conditions 
and methods across the world, a thick 
seam is often defined as a seam whose 
full thickness cannot be efficiently ex- 
tracted using the available equipment. 
In the Western United States, the maximum 
extractable mining height is 14 ft. For 
this report, a thick seam is defined as 
any minable coal seam 15 ft high or 
higher. Closely spaced or split seams 
are also included in the report, as one 
of the two seams is usually lost from the 
reserve base due to subsidence of the up- 
per undeveloped seam when present-day 
mining practices are used. 

Because of the manner in which coal re- 
source figures have been compiled, it is 
difficult to estimate the western re- 
source base for coal seams greater than 
15 ft thick or for minable seams split by 
a parting. For this reason, the author 
chose to look at "occurrences" in three 
States instead. Colorado, Utah, and Wy- 
oming (fig. 1) were evaluated for re- 
ported occurrences of thick seams at 
3,000 ft or less of overburden. Although 
data available to the author were more 
limited for Utah and Wyoming, both States 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



did have a substantial number of occur- 
rences reported in publications summariz- 
ing their coal resources. In Colorado, 
the accessible data were more extensive, 
consisting of over 1,500 records on inac- 
tive coal mines in the State and ba- 
sin analyses on major coal regions that 




200 

I 



LEGEND 

I I Coal-bearing region 

QE Area known to contain minable thick 

or very closely spaced coal seams 



FIGURE 1.— Map of coal-bearing regions in Colorado, Utah, 
and Wyoming; adapted from Trumbull (2). 



included listings of coal "calls" from 
oil and gas logs. Given the foregoing 
data density, the occurrence of thick 
coal seams in Colorado and past develop- 
ment in those seams were better charac- 
terized for Colorado than for the other 
two States. 

Table 1 lists the coal regions in Colo- 
rado that have reported occurrences and 
past or present mining activity in thick 
coal seams. The limited available infor- 
mation for Utah and Wyoming is also in- 
cluded. The numbers represent separate 
occurrences spread over a large area. 
In the past, any development at these 
points often resulted in a loss of ton- 
nage from the reserve base because of 
incomplete extraction. Without a change 
in the mining method, the same will be 
true in the future. The mines given in 
table 1 include both historic and modern 
operations. 

Table 2 lists active or proposed under- 
ground mines in thick coal seams in the 
the three States (3-4). It should be 



noted that Wyoming is known for its thick 
seams which, in places, exceed 200 ft. 
In the past, many of the mines produc- 
ing from thick seams were underground. 
At present, only one active underground 
mine is in a thick seam; the remaining 

TABLE 1. - Summary of thick- and split- 
seam occurrences and development in 
Colorado, Utah, and Wyoming 



Region or basin 


Occurrences 


Mines 


Colorado: 


6 

68 

16 

121 


3 




9 
15 


Uinta 


42 








211 
52 
36 


69 
ND 
ND 







ND Not determined. 



Available data 



were inconclusive on the number 
that have, in the past, worked 
or split coal seams. 

'Undifferentiated by region. 



of mines 
in thick 



TABLE 2. - Known active or proposed underground mines working 
thick or split seams in Colorado, Utah, and Wyoming (4) 



Region 



Mine 



Seam 



Thickness, 
ft 



Colorado: 

Canon City 

Green River 

Do 

Uinta 

Do 

Do 

Do 

Do 

Do 

Do 

Do 

Do 

Utah: 

Book Cliffs 

Do 

Do 

Kaiparowits Plateau. . . 
Wasatch Plateau 

Do 

Wyoming: 

Hanna 

'Multiple seams — minimum 



Dorchester No. 1. 

Eagle No. 5 

Little Bear Creek 
Coal Ridge No. 1. 

Deserado 

Dutch Creek No. 2 

Fruita No. 1 

King 

McClane Canyon. . . 
Munger Canyon. . . . 
Orchard Valley... 
Somerset 

Price River Coal. 
Soldier Creek. . . . 

Sunnyside 

John Henry 

Skyline 

Valley Camp 

CCCC No. 1 

Van guard No. 2 . . . 
of 2. ^~3 seams. 



Red Arrow and Dirby Jack' 

F 

Seymour 

Wheeler 

A, B, and C 1 

Dutch Creek 

Cameo' 

B 

Cameo 

...do 

D 

B 

Castlegate' 

Rock Canyon' 

Sunnyside' 

A and B 1 

' Conner 2 

. . .do 

No. 80 

No. 50 and No. 51 



5-6 

8-22 

15 

42 

0-17 

4-20 

10-25+ 

16 

17-22 

5-26 

4-27 

14-21 

5-12 
13 
4-12 
6-18 
4-24 
0-24 

20+ 
4-22 



operations are surface mines. In the 
future, as economic stripping ratios 
are exceeded, the surface operations in 
Wyoming may again move underground, 



necessitating the incorporation of more 
efficient extraction methods to develop 
the thicker seams economically. 



THICK-SEAM MINING METHODS 



In the Bureau's research on thick-seam 
mining, four methods are being investi- 
gated for use in western thick-seam 
mines: (1) high-face single-pass long- 
wall, (2) multislice longwall, (3) long- 
wall caving, and (4) hydraulic mining. 
The following brief summaries of the 
first three methods are derived from 
Oitto (5). Data on hydraulic mining were 
taken from information published by Kai- 
ser Resources (6) . 

The high-face single-pass longwall 
method is set up similar to a standard 
longwall, the main difference being ex- 
traction height. While a standard face 
may be 7 to 9 ft high, a high-face long- 
wall extracts up to 18 ft of coal through 
the use of specialized equipment. The 
high-face longwall method was developed 
for coals dipping less than 25°. 

The second method, the multislice long- 
wall method, mines thick seams in slices 
or lifts (fig. Ik ) • Mining may be simul- 
taneous (concurrent mining) with two or 
more lifts being worked, or nonsimultane- 
ous (nonconcurrent mining) where all pan- 
els in the upper lift are mined first, 
one at a time, before proceeding to a 
lower lift. This method is practical in 
coals from 14 ft to greater than 100 ft 
thick. The method is especially adapta- 
ble to thick seams that contain partings 
or other impurities that make part of the 
seam uneconomic. Seams separated by less 
than 30 ft of interburden will probably 
require multislice mining methods. 

Longwall caving is a hybrid that com- 
bines traditional longwall mining and 
sublevel caving mining methods. In this 
method, a 7- to 10-ft slice is made at 
the base of the coal seam, and the re- 
maining upper part of the seam is allowed 
to cave downward (fig. 2B). The caved 
coal is drawn through the shields by us- 
ing gates or chutes set on the gob side 
of the shields and transported from the 
longwall face. The method was developed 



for coal seams 18 ft thick or greater. 
In very thick coal seams , longwall caving 
can be done in successive 30-ft slices. 

Hydraulic raining is a method used to 
extract thick, dipping (greater than 4°) 
coal seams. This system extracts coal 
from retreat panels using hydraulic moni- 
tors which discharge water at high pres- 
sure from nozzles. Coal dislodged by the 
water is washed into feeder-breakers and 
then into an inclined flume for gravity 
transport. Advantages of this method may 
include mechanical simplicity, dust con- 
trol, and safety (due to removal of work- 
ers from the face areas since the con- 
trols are often located 20 to 40 ft outby 
the face). 




,4, Multislice 













mm 



J o 



Waste® 



B, Longwall caving 

FIGURE 2.— Two thick-seam mining methods. 



GEOLOGY AS A SCIENTIFIC TOOL 



Many features that affect the develop- 
ment of a mining property are geologic in 
nature. The presence of these features 
can be delineated by a thorough geologic 
analysis of the property prior to devel- 
opment. Due to geology's interpretive 
power, it is an essential tool in setting 
up a mine property; however, this tool is 
not often used to its fullest capacities 
during mine development. 

The geologic analysis of a coal prop- 
erty for development begins during the 
first stages of an exploration program. 
During exploration, the structure and 
stratigraphy are determined for an area. 
This often includes a widespread evalua- 
tion of a part of the coalfield to pick 
up general trends, followed by site- 
specific studies of the properties of 
interest. The exploration phase deter- 
mines the location, continuity, thick- 
ness, change in thickness, and quality 
of prospective coal mining targets. Mul- 
tiple methods of obtaining data are 
used — surface mapping, aerial photogra- 
phy, drill and coreholes — all of which 
help to give a three-dimensional view of 
the subsurface. Other methods, such as 



seismic, gravimetric, magnetic, and other 
surveys, have their place in filling in 
the unknowns in this subsurface view. 
It is during exploration that the first 
idea is formed of what geologic hazards a 
thick-seam mine will face during its 
development; although these hazards may 
not be fully realized until mining is 
underway. 

After development begins, the geologist 
can obtain data from another point of 
reference — inside the developing mine. 
In-mine mapping and subsurface coring al- 
low the geologist to fill in information 
and further verify trends and features 
seen in exploration. 

The following section describes factors 
that need to be examined and, as far as 
possible, quantified. Many of these fac- 
tors are affecting thick-seam development 
on the international level and will prob- 
ably affect the success of thick-seam op- 
erations in the Western United States. 
The process of subsidence is important in 
thick-seam mining, but it is not within 
the scope of the paper to discuss this 
topic. 



GEOLOGIC FACTORS AFFECTING THICK-SEAM DEVELOPMENT 



The geologic factors affecting thick- 
seam development are often the same fac- 
tors that affect standard longwall devel- 
opment. In thick-seam mining their dele- 
terious effects may be magnified owing to 
a greater face height. 

CLEATS 

Cleats are essentially fractures in 
coal. They are usually present in a set 
of two: the face or primary cleat, and 
the butt or secondary cleat. The two 
cleats are usually 90° to each other, but 
this angle may increase or decrease, due 
possibly to deformation of the seam. 
Cleat surfaces are usually smooth and 
planar, yet like the surfaces of frac- 
tures of joints, they can also be rough. 
Mineralization is common along the 
cleats. 



In the past, the presence of cleats was 
essential to mine development. The coal 
was mined either parallel or perpendicu- 
lar to the cleat, so that it broke away 
from the face during mining (7^). Contin- 
uous and longwall mining methods , when 
they were introduced, allowed for effi- 
cient extraction of coal independent of 
the cleat orientation to the coal face. 
Nevertheless, the cleat and its orienta- 
tion and development are still important 
factors to consider in mine layout, es- 
pecially in thick-seam mining owing to 
the increase in mining height and surface 
area at the face. The characteristic 
that will have the greatest effect on 
thick-seam mining is cleat development, 
or, the degree of formation of a cleat 
surface, which is dependent on many fac- 
tors: (1) the maceral makeup of the coal 



(.8), (2) the degree of coalif ication, and 
(3) the content and composition of ash 
within the coal (9^» 

In a coal seam, the development of the 
face and butt cleats varies. The face 
cleat is, by standard definition, the 
more continuous, well-developed, fracture 
that cuts across bedding planes within 
the coal. The butt cleat may or may not 
be as well developed as the face cleat. 
The degree of development of both sur- 
faces has a direct effect on thick-seam 
extraction in three major areas: 

1 . Cleat development in the coal de- 
fines the macropermeability or fracture 
permeability of the seam. The variable 
development of the face and butt cleats- 
causes the coal to be anisotropic in 
its transmission of water and gas. This 
would have a direct effect on their move- 
ment at the face and in the panel(s) be- 
ing mined. 

2. According to Vaninetti (10) , "If 
cleats are prominent, then strata pres- 
sure on the coal face often results in 
slabbing." This slabbing causes jams at 
the face conveyor and stage loader trans- 
fer points (5 ) , which increases downtime 
and decreases productivity. Also, the 
higher the face or rib, the larger the 
slab will be, which in turn increases the 
hazard to miners in the working sec- 
tion. Vaninetti (10) also noted that, 
due to slabbing, the stability of entries 
and faces can be reduced. In the same 
line, Bottrill noted (11) that faces at 
right angles to the cleat show a greater 
tendency to burst conditions than faces 
parallel to the cleat, where constant 
"spalling" of the coal relieves the 
situation. 

3. In longwall caving, the development 
of cleats is critical, as their develop- 
ment coupled with their density can be a 
determinant of size of caved material 
dropped through the caving chutes. 

Just as cleat development is important, 
changes in the development of the face 
and butt cleat through the seam can 
determine the success of a mining method, 
especially longwall caving. Cleat devel- 
opment may vary due to changes in maceral 
composition, the presence of partings, or 
possibly the deformational history of the 
seam. As an example, cleat development 



was logged on the core of a 202-ft sub- 
bituminous coal seam (fig. 3). The data 
(12) showed a change in cleat development 
down section in the Anderson deposit; 
there was poor to no cleat development in 
the upper 69 ft, increasing downward to 
poorly to well developed cleats in the 



CORE OF 

ANDERSON 

DEPOSIT 



r- 1030 



- 1,100 



- 1,050 



CLEAT DEVELOPMENT 

FACE BUTT 



- 1,150 



-1,200 




1,250 



FIGURE 3.— Generalized log of corehole showing depth and 
degree of cleat development throughout the core. Partings 
designated by dashes. For cleat development, N = none, 
P = poor, M = moderate, W = well. Adapted from Boreck {12). 



lower 133 ft of the seam. Oitto (5_) 
noted that longwall caving was suitable 
for friable coal that caves naturally by 
gravity. Given the above example, the 
upper 69 ft of the seam may not cave 
spontaneously during caving unless frac- 
tures other than the cleats are present. 
This would necessitate the use of explo- 
sives or other methods to induce caving 
and decrease block size of the caved ma- 
terial. On the other hand, a thick coal 
with well-developed cleat sets near the 
top may be more prone to cave spontane- 
ously. For this reason, it is important 
to determine not only cleat development 
but also changes in both the development 
and density through the seam. 

JOINTS AND FRACTURES 

Joints are similar to cleats in that 
they are vertical to near-vertical frac- 
ture sets. They can occur singly; but 
are more commonly found in two or more 
sets, distinguishable from each other by 
dissimilar orientations. The difference 
between joints and cleats is that joints 
are found predominately in rock. 

The main characteristics of joints that 
will affect thick-seam mining are devel- 
opment, orientation, and spacing. Well- 
developed joints, depending on their con- 
tinuity, act as vertical zones of 
Weakness at which failure can occur. 
Their presence can be detrimental, where 
roof joints at the face allow for pre- 
mature movement in response to stress 
on the longwall panel, or beneficial, 
where joints allow a strong roof to cave 
easily. 

The joint orientation is as important 
in thick-seam mining as it is in stan- 
dard mining. The difference between 
joint orientation and the orientation of 
the longwall face and gate entries will, 
in part, determine the stability of the 
roof and its cavability. The orienta- 
tions of surface joint sets have been 
used to predict the orientation of sub- 
surface joints, as have lineation studies 
of the property and analysis of major 
structural trends in the area. The pos- 
sibility of error in utilizing these 
methods varies with the property and its 
geologic history. 



The joint spacing is important in that 
it, in part, determines the competency of 
the roof rock and the block size of the 
caved material. The spacing can vary 
substantially, causing changes in roof 
conditions across a panel. Joint spacing 
in the subsurface often cannot be effec- 
tively predicted unless the roof rock is 
exposed and mappable within the mine. 

Joints can be open or closed, have 
smooth or rough surfaces, and be filled 
with minerals such as calcite, pyrite, 
gypsum, or clay. These characteristics, 
in part, determine the degree of weaken- 
ing of roof rock above the seam. Joints 
act as conduits for water and gas enter- 
ing the mine workings. 

Fractures are breaks within the coal 
and surrounding strata that are not di- 
rectly related to either cleat or joint 
sets. For this report, breaks both with 
movement on the plane of separation 
(slickensides) and without such movement 
will be considered as fractures. Frac- 
tures that displace the coal (faults) 
are considered under another heading. 

Fractures can result from tectonic ac- 
tivity or postdepositional compaction. 
They may or may not be vertical. The 
fracture planes can be flat or curved and 
can have variable orientations. Frac- 
tures, like joints, can be open or 
closed, have smooth or rough surfaces, 
and be mineralized. They may also act as 
conduits for water and gas. 

The effects of high-angle fractures are 
similar to those of joints and cleats. 
Low-angle fractures, because of the shal- 
low dip, can separate prematurely during 
the mining process. This can be a crit- 
ical problem in hydraulic mining (13) . 
Individual fractures are often localized 
discontinuous feaures, although a group 
or zone of fracturing may extend for a 
long distance, as in differential compac- 
tion features (slickensides or slips) 
that may parallel a sandstone channel. 

COAL QUALITY 

Coal quality is one of the most subtle, 
yet important, controls on development of 
a thick-seam mine. In the past, quality 
helped determine which part of the seam 
was mined. High ash or sulfur zones 



ruled out mining parts of a thick seam or 
one of the two closely spaced coal seams. 
Changes in quality through a seam may 
determine the mining method. A thick 
seam containing partings or bone near the 
center is amenable to development using 
a multislice longwall setup, especially 
where mining the parting or bone in- 
creases the ash content above acceptable 
limits. 

One example of the control of quality 
is the Rienau property in the Danforth 
Hills Coalfield, western Colorado. The 
seam worked on the property is the Rienau 
Bed. The seam reportedly ranges from 11 
to 24 ft thick and dips at 18° ( 1_4 ) . The 
thick-seam property was first developed 
in 1928 and has been worked sporadically 
since that time. The mine was originally 
designed to accommodate a bone split 
(parting) that occurred two-thirds of the 
way up the seam. Rooms were driven at 
the top and bottom of the coal, avoiding 
the split. Given these conditions, the 
multislice method may be the most effec- 
tive means of increasing recovery in 
mines similar to Rienau property, while 
at the same time keeping the product's 
quality high. 

PETROGRAPHY 



combust. Bacharach (15) reported that 
petrographic consituents differed in 
their tendencies to combust. The exinite 
maceral was reported as having a higher 
oxidation rate than the vitrinite or in- 
ertinite macerals, while fusain was the 
least reactive. The maceral content of 
the coal and the relative abundance of 
the more reactive macerals help determine 
the liability of a seam or parts of a 
seam to spontaneous combustion and help 
explain why some seams will catch on fire 
while others do not. 

Also of major importance is the maceral 
makeup's effect on the mining of a thick 
seam. An example of this condition was 
given by Ahcan (1_6) in his description of 
a longwall operation producing lignite in 
Yugoslavia. Ahcan noted that, for the 
lignite, breakage often decreased and 
blasting of the coal was necessary. One 
reason given for the coal's tenacity was 
the presence of xylite intercalations in 
the lignite. Xylite, macroscopic bands 
within brown coals derived from stumps 
and stems that have been vitrified (17) , 
increased the lignite's strength by ap- 
proximately 15 pet. This increase, along 
with other factors, resulted in a higher 
consumption of explosives to stimulate 
caving. 



The petrographic composition of the 
coal also plays a subtle but important 
role in thick-seam mining. The maceral 
type and percentage can affect cleat 
development. Coals rich in a certain 
type of maceral will have a substantially 
different cleat density than coals rich 
in other macerals. Vitrinite bands in a 
bright coal commonly have a higher cleat 
density than the constituents of a dull 
coal. Dull coal (composed of fine- 
grained exinite, inertinite, vitrinite, 
and fine-grained disseminated mineral 
matter) tends to be hard, compact, and 
blocky (8). Cleat density, in turn, 
helps determine the friability of the 
coal, its tendency to cave (effects of 
block size), and its permeability to 
gases and water. 

The petrographic makeup of the coal has 
a subtle, yet important effect on the 
tendency of the coal to spontaneously 



ROOF LITHOLOGY 

Lithology of the roof is an essential 
consideration for any mining method. In 
planning a thick-seam mine, an analysis 
of the roof lithology should contain in- 
formation in three major areas: (1) a 
description of the rock type or types 
present in the roof, (2) documentation of 
vertical changes in the roof (bedding, 
low-angle planes of separation, rooted 
zones, and abrupt horizontal or low-angle 
changes in lithology), and (3) evaluation 
of lateral changes in rock type, the na- 
ture of the contacts between different 
roof rock, and the presence, orientation, 
and development of high-angle fractures 
and joints, over the panels to be mined. 

The roof rock in western coal mines can 
often be highly variable. Roof lithology 
is determined by the type of sedi- 
ments deposited under changeable energy 



regimes involving the termination (often 
followed by the reestablishment) of peat 
deposition. Some of the most common 
western rock types and their effects on 
mining follow: 

Sandstone . — A massive sandstone roof 
is, in most cases, the strongest, most 
competent roof type in western coal 
mines. It is a good roof for development 
mining and for main entries that need to 
stay open for transport of coal, equip- 
ment, and personnel. However, for most 
other phases of thick-seam longwall min- 
ing, the sandstone roof makes a poor 
roof. 

Due to their competency, sandstones, as 
a rule, do not cave well in longwall or 
retreat mining. Ghose (18) reported on a 
mine using thick-seam single-pass long- 
wall development in Czechslovakia. The 
immediate roof in the mine was made up of 
laminated sandstone with alternating con- 
glomerate beds. The roof type was char- 
acterized by difficult caving and a lia- 
bility for bumps. Khanna (19) noted in 
his study that the caving of an immediate 
sandstone roof was fraught with dangers 
such as crushing, collapse of pillars, 
and airblasts due to delayed caving of 
the competent roof. 

The sandstone body may also be quite 
variable internally. It can be thick, 
strong, moderately homogeneous, and rel- 
atively massive. Or the sandstone can be 
weakened by poor cementation, the pres- 
ence of shale lenses or coal stringers, 
pebble lag, well-developed crossbedding, 
or thin flaggy sandstone beds, often 
found at the base of the sandstone. Ow- 
ing to their relatively high permeabil- 
ity, sandstones are potential aquifers 
and both store and transmit water (and 
gas). Even if the sandstone is not in 
direct contact with the mined coal, frac- 
tures in the roof can bring water and 
gases into the mine. 

Shale . — Shale is a laminated sediment 
made up predominantly of clay. It is of- 
ten characterized by fissility. A shale 
roof can be either a good or bad roof in 
all mining methods. Thick sequences of 
shale without fractures, slickensides, or 
organic debris can make a good roof. It 
caves easily — an advantage in longwall 



as a 
Be- 



mining. Major problems with shale are 
its fissility and tendency to slake when 
exposed to humidity. 

Claystone . — Claystone is defined 
rock consisting of indurated clay, 
cause of the high clay content, the rock 
tends to soften and decompose in the 
presence of excess water. Moebs (20) 
noted that claystone makes poor roof when 
it is massive and unlaminated. 

At least one thick-seam mine has re- 
portedly experienced problems with the 
presence of claystone in the mined sec- 
tion. Ahcan (21) reported that, in the 
Kreka Lignite Basin, the longwall face 
mining methods were not successful be- 
cause the lignite seams, ranging up to 
10 m thick, were "inbedded into very soft 
clay layers." 

To evaluate the cavability of the roof, 
researchers have recommended analyzing 
the properties of the roof rock up to 
four times the thickness of the seam to 
be mined (22). The geologic considera- 
tions are not only the type of roof rock, 
but changes in the rock vertically above 
the seam and laterally across the panels. 
The thickness of individual layers or 
beds making up the roof strata should 
also be included. Figure 4 is a north- 
south cross section through a thick coal 
in western Colorado. The section shows a 
two-dimensional representation of litho- 
logic changes vertically and laterally in 
the roof, coal (including partings), and 
floor. Vertically, the lithology of the 
roof is variable. Horizontal zones or 
planes of weakness (H) occurring along 
bedding planes, contacts between differ- 
ent lithologies (as in coreholes A-D) , 
rider seams and coal stringers (core- 
holes A, C, D) , rooted zones, and layers 
of carbonized plant material (coreholes 
A, C) are common throughout the section. 

Workings in the center of the seam 
would be subject to roof failure owing 
to the presence of partings in the up- 
per part of the seam. Rider coals and 
stringers in the roof, besides becoming 
zones of weakness, will also contribute 
to the total gas emission that must be 
handled in gassy thick-seam mines. 

The lateral lithologic changes in fig- 
ure 4 are as variable as the changes 



10 



South 



North 




H=lO 



Scale, ft 



FIGURE 4.— Cross section (coreholes A-D) through thick coal in the Danforth Hills Field, western Colorado. H = horizontal 
planes of weakness; V = vertical planes of weakness. 



through a vertical section. For this ex- 
ample, low- to medium-angle fractures 
(V), although not evident from the logs, 
are also hypothesized to be present. 
Thick sandstone would possibly make a 
competent roof in the area intersected by 
section B. Yet, the roof adjacent to the 
sandstone, because of the large number 
of horizontal zones of weakness (H) and 
postdepositional compactional features 
(V) that often develop under and adjacent 
to sandstone channels, may be unstable, 
caving easily (possibly before it should 
cave). 

FLOOR LITHOLOGY 

Evaluating the lithology of the floor 
rock is often not emphasized as much as 
evaluating the roof. However in a long- 
wall operation, the floor must bear the 
weight of the shields and their load, 
and its lithology and three-dimensional 
changes in lithology need to be evalu- 
ated. The strength of the floor, the 



reactivity of the immediate floor to 
water or increases in humidity, and the 
presence of gas-bearing coal seams or 
gas- or water-bearing strata in the floor 
must be determined. 

THICKNESS AND THREE-DIMENSIONAL 
CONFIGURATION OF COAL SEAM 

The majority of western coal seams 
are lenticular bodies. The seams can 
vary significantly in thickness within a 
single mining property. An example was 
given by Ryer (23) . The thickness of the 
I coalbed, of the Ferron Sandstone Mem- 
ber, Emery Coalfield, Utah, shows a lat- 
eral change of approximately 23 ft over 
a distance of 1.2 miles. Less signifi- 
cant changes can also be present in a 
coal seam as a result of depositional 
basin topography or postdepositional com- 
paction. Depending on the seam thick- 
ness, it is critical that changes in the 
thickness across a property be deter- 
mined prior to selection of a mining 



11 



method. This ensures an adequate match 
between equipment and thickness of avail- 
able reserves. 

CONTINUITY OF THE COAL SEAM 

Continuity of the coal seam is related 
in part to its configuration and in part 
to features that displace or thin it as a 
result of structural and depositional 
controls. Three features that are impor- 
tant in thick-seam mining are faulting, 
washouts, and partings. 

Faults cutting the coal and displacing 
the mined seam cause the same problems in 
thick-seam development as they cause in 
standard mining: (1) displacement of the 
coal seam, limiting reserves, (2) frac- 
turing and weakening of the coal and 
surrounding rock, and (3) migration path 
for water and gas into the mine workings. 
The effects may not be critical to long- 
wall caving or hydraulic mining unless 
the coal and roof are highly fractured 
over a large area. The effects would be 
more evident in multislice or single-pass 
mining, where a slight displacement may 
throw the panel off. If the fault was 
active during deposition of the coal, the 
area may be marked by an abrupt thinning 
or thickening of the coal seam. In the 
West, some faults may be located by sur- 
face mapping and by using remote sensing 
techniques. 

Smaller faults (growth faults or slump 
features) are more subtle (fig. 5) and 
may not be detectable until they are 
encountered in the mine. Their effects 
would depend on the amount of strata dis- 
placement and orientation of the faults 
with relation to the longwall face. 

Depositional features that hinder mine 
development are similar in both thick- 
seam and standard mining. As in fault- 
ing, the percentage of the seam affected, 
its orientation, and the lateral extent 
of the features determine the effects 
on face development. A feature related 
to deposition is a washout (channel 
erosion) . 

Washouts, or wants, are defined as 
areas where the coal has been eroded by a 
channel above the coal seam, often re- 
sulting in rapid thinning of the coal. 



The coal seam may be partly or completely 
cut out by the channel. The effects 
on thick-seam mining would depend on 

(1) the mining method being utilized, 

(2) the initial thickness of the seam, 
and (3) the percentage of the seam eroded 
or missing. An unexpected decrease in 
seam thickness may not bring a longwall 
caving operation to a standstill, yet 
the multislice and high-face single- 
pass longwall methods would be seriously 
affected. Channels may or may not be 
located during preliminary exploration. 
Areas believed to be affected by channels 
require intense exploration to evaluate 
the extent of the washouts. 

The above two features are important to 
thick-seam development. A third factor, 
the presence of a parting or split in the 
seam, is critical to thick-seam mining; 
the parting alone may determine the min- 
ing method used. 










FIGURE 5.— Example of small fault (growth type). 
Downthrown block to the right. 



12 



Thick seams split into two or more 
seams are common in the Western United 
States. The parting occurs when deposi- 
tional processes interrupt coal swamp 
development. This usually affects one 
part of the swamp. The swamp then rees- 
tablishes itself on the newly deposited 
sediments. As a result, the coal is 
split into two or more seams separated by 
from <1 ft to >30 ft of rock parting. 
The depositional model helps determine 
the thickness and aerial extent of the 
coal seams and their interburden, charac- 
teristics that are essential considera- 
tions in determining the feasibility of 
adopting any of the subject methods. 

One of the methods most adaptable to 
the above condition is the multislice 
method. Incorporation of multislice min- 
ing requires that substantial minable 
reserves exist in both the upper and 
lower seams. Both seams need to be 
relatively continuous without appreciable 
faulting or washouts. The lithologic 
makeup of the parting is important in 
that it is the floor of the upper lift 
and the roof of the lower lift. 



cannot be adequately predicted during the 
exploration phase prior to mining. 

Although the rolls studied in mines are 
usually small-scale features in coals of 
standard thickness, Law (24) presented 
proof of large-scale compactional fea- 
tures in the coal-bearing Fort Union and 
Wasatch Formations. Law noted that the 
structures were the result of differen- 
tial compaction. Law also wrote that the 
magnitude of the folds had been intensi- 
fied by the unusually thick coal in the 
section. 

Similar features were noted by the 
author in parts of the Anderson Deposit 
(fig. 6). In figure 6, a domal structure 
(shaded area) is present. The most ob- 
vious problem would be an increase in 
grade or dip of the seam, a problem that 
could be dealt with in mine planning. A 
second, less obvious problem could be the 
effect of the feature on fluid flow 
through the strata, causing a buildup of 



~l 



ROLLS 

Rolls, as they occur in the West, have 
been defined by Vaninetti (10) as small- 
scale folds in coal and enclosing strata 
formed in response to differentially com- 
pacted sandstones (or other lithologies) 
pushed into the coal during compaction. 
The effects are seen in local changes in 
grade in the coal seam with possible 
slight thinning or thickening of the coal 
associated with the roll. In a thick 
coal seam, the presence of the small- 
scale rolls may not cause significant 
problems during mining. But, in split 
seams, the unexpected presence of rolls 
can cause significant damage. Oitto (_5) 
noted that multislice longwall mining 
requires at least 14 ft of coal for two 
longwalls to progress at different eleva- 
tions without being forced out of their 
horizons by undulations in the roof or 
floor. The existence and trend of rolls 
or other smaller scale features usually 




^ H 



10,000 20,000 



J 



FIGURE 6.— Structure map drawn on top of coal seam in 
part of the Anderson Deposit. Shaded portion denotes area of 
structural interest. Contour interval is 100 ft. 



13 



gas and water in different parts of the 
structure. 

SPONTANEOUS COMBUSTION 

Spontaneous combustion is a critical 
factor that needs to be considered in 
western coal development; it has caused 
and continues to cause a substantial 
loss of minable reserves. Western thick 
seams often have a high susceptibility 
for spontaneous combustion. A main rea- 
son for this susceptibility is the pres- 
ence of coal remaining in the section 
during or after mining. 

The process of spontaneous combustion 
is complicated, involving both geologic 
(seam thickness or presence of multiple 
seams, rank, sulfur content, petrography, 
moisture, particle size, and caving char- 
acteristics) and mining factors. Several 
of the geologic factors that directly af- 
fect ignition potential in western thick 
seams are discussed below. 

Seam thickness is an important consid- 
eration in spontaneous combustion (15). 
Where seam thickness is greater than the 
mining height, excess coal left in the 
workings has a high potential for igni- 
tion. The thicker the coal, the greater 
the volume of coal remaining in the mine 
and the higher the risk is for spontane- 
ous combustion. For split seams, Bach- 
arach (15) noted that — 

Where a multi-seam situation ex- 
ists, both during the working of 
the first seam or subsequent seams, 
situations can arise with spontane- 
ous combustion hazards for the seam 
currently being worked and any 
other seam above or below it. For 
example, where a seam has been 
worked with another unworked seam 
underlying it, leakage paths can be 
created into the lower seam, with 
consequent risk of heating. In 
other circumstances, where a seam 
is worked under an overlying un- 
worked or worked seam, the later 
mining operations can result in 
fire hazards in the upper seam. 
Rank can be an important variable 
in determining ignition susceptibility. 



Although coals of any rank are subject to 
spontaneous combustion, lower rank coals, 
due to a higher tendency to oxidize, are 
more prone to spontaneous Ignition than 
the higher rank coals. Many of the thick 
coals in the Western United States are 
lower rank subbituminous coals. As a 
result, fires in surface and underground 
mines occur frequently (fig. 7). 

Moisture content in the coal is consid- 
ered to be an essential factor in sponta- 
neous combustion in the West. Bacharach 
(15) discussed how changes in moisture 
content can lead to heating. The mois- 
ture content of the unmined coal and its 
surrounding environment is in a relative 
state of equilibrium. With mining, an 
increase in moisture content in the air 
and a higher vapor pressure can lead to 
absorption of water and a rise in temper- 
ature in the coal. Dunrud (25) in his 
studies on underground fires in abandoned 
mines noted that fires may ignite by 
spontaneous combustion where air and wa- 
ter reach exposed coal through subsidence 
cracks, pits, or open or poorly sealed 
mine portals. 

The area exposed to air and water is 
another factor determining the liabil- 
ity of the coal to ignite. Exposed sur- 
face area increases with increased mining 
height and friability of the coal. The 
friability may be due to the petrographic 
makeup of the coal and the presence of 
faulting or intense fracturing in the 
coal. 

Other factors besides the above men- 
tioned that affect the ignition potential 
of the western coal seam will not be 
covered. Some, like the geothermal gra- 
dient, may be more important in some 
areas (like Colorado) than others. Min- 
ing factors such as mining method, rate 
of advance, and mine layout also affect 
the potential for sponteneous ignition to 
take place (15). 



GAS 



Explanations as to how gases form and 
migrate are complex, and gas continues to 
be a main threat to miners working under- 
ground. The gases commonly encountered 



14 







FIGURE 7.— Fire in strip operation in southern Colorado. (Courtesy Colorado Geological Survey) 



underground are CH 4 , C0 2 , CO, H 2 , N 2 , 
and H 2 S. This section will discuss the 
effects of CH 4 on thick-seam develop- 
ment, although the other gases, naturally 
occurring or otherwise, can have equally 
deleterious effects. 

Methane (CH 4 ) is a major constituent of 
natural gas, occurring as a byproduct 
during the decay and upgrading of carbon- 
aceous material. Although the maximum 
yield of gas from coal occurs in the 
high-volatile A to low-volatile bitumi- 
nous range, CH 4 can still be present in 
lower rank coals. This point is critical 
in the discussion of western thick seams 
because many of these seams are low rank. 
Reported occurrences of CH 4 in water 
wells and gas problems or blowouts on 
coal exploration rigs are common (26) ; 
often the rigs are drilling at relatively 
shallow depths. The gas is associated 
not only with the coal, but also with 
permeable sandstones within the coal- 
bearing section. Core desorption from a 



202-ft-thick seam (part of the Anderson 
deposit) resulted in gas contents of 56 
to 74 std ft 5 per short ton of in-place 
coal. Although lower than expected for a 
higher rank coal, the gas content coupled 
with the size of the source and reservoir 
can cause difficulties during development 
of a thick-seam operation. 

The presence of CH 4 (or any other gas) 
in thick-seam mines can cause multiple 
problems. One such problem was noted by 
Bise ( 27 ): 

In thick-seam workings, particu- 
larly during the phase when the 
whole seam is extracted, the wide 
and high roadways create conditions 
which may lead to low air veloci- 
ties. In gassy seams, methane lay- 
ering along the roof may result. 
Another problem that can be encountered 
in a thick gassy coal seam is a gas out- 
burst. The outbursts can be caused by 
CH 4 , C0 2 , or other gas under pressure 
within the coal. 



15 



The Bureau has been highly involved in 
the study of methane and its genesis 
and migration since the early 1970' s 
(28). This research, along with previous 
research done by the Bureau and other 
organizations, has resulted in a substan- 
tial number of tools that assist in de- 
termining the quantity of gas, its qual- 
ity, and controls on migration through 
the coal and rock into the mine. The 
tools are as usable on thick-seam prop- 
erties as they are in seams less than 15 
ft thick. The projected gassiness of the 
mine can be determined utilizing standard 
desorption methods (28). Probable migra- 
tion paths can be delineated by an analy- 
sis of orientation of cleats. In thick 
seams, it is known that cleat development 
may change throughout the seam's thick- 
ness. To what extent this would affect 
gas emissions in thick-seam mining is not 
known. It is important to document all 
gas sources and their content and the 
degree of cleat development prior to mine 
development. 

WATER 

Water is a significant concern in any 
underground mining operation, and the 
problem is magnified in thick-seam opera- 
tions owing to increased seam thickness 
and the tendency for some western coals 
to act as aquifers. In the West, water 
is an essential resource, as was pointed 
out by Van Voast (29) in his report on 
the hydrologic characteristics of coal 
mine spoils in southeastern Montana. The 
report dealt with the coal-bearing Fort 
Union Formation. The uppermost unit of 
the formation, the Tongue River Member, 
contains 26 coal seams with thicknesses 
ranging from 3 to 78 ft. In the report, 
Van Voast noted — 

Energy is not the only resource 
that southeastern Montana can pro- 
vide. The coal beds, because of 
their generally fractured nature 
and large areal continuities, are 
commonly the most accessible and 
widely used aquifers of the region. 
In this semiarid climate, many in- 
habitants are almost totally de- 
pendent upon ground water for stock 
and domestic supplies and in many 



places they obtain it (water) from 
coal beds that will be removed by 
mining. 

The coalbeds from the Tongue River Mem- 
ber had transmissivities of 0.4 to 41.0 
m 2 /d. Often they were not considered 
viable wells by standards set in nonarid 
environments. Yet, the wells are con- 
sidered essential to the inhabitants of 
an area where other easily obtainable 
water sources do not exist. 

Water can be a hazard in thick-seam 
mining, whether it drains from surface 
sources through subsidence cracks or 
fractures, is tapped into from confined 
aquifers in either the overburden or 
strata directly under the coal, or is de- 
rived from the mined seam. Ahcan (30) 
discussed the effects of water collecting 
in gob areas during use of the vertical 
concentration method (longwall caving of- 
ten combined with multislice mining). In 
Yugoslavian mines, inrushes consisting of 
water or mud occurred during winning 
from the overhanging face section. Ahcan 
noted that the water inrushes caused min- 
imal difficulties as compared to the mud 
inrushes. The mud, resulting from a mix- 
ture of water and clay originally de- 
rived from the coal and overlying strata, 
flowed into the face area, causing casu- 
alties and damage to the longwall. Ahcan 
also noted that all the mud inrushes and 
many of the water inrushes occurred 
at faces using the vertical concentra- 
tion method. The horizontal concentra- 
tion method (standard longwall mining 
with a face height of approximately 10 
ft) was considered safer where imperme- 
able clay strata separating the workings 
from water-bearing formations in the roof 
were too thin to act as a barrier. 

Another example demonstrated the effect 
of water below the mined seam. Strong 
(31) noted that in the South Wales Coal- 
field, coal was mined above a highly 
jointed limestone aquifer. The water in 
the aquifer was artesian. As such, early 
excavations in the field experienced in- 
rushes of water from the aquifer when the 
intermediary strata were disturbed. 

The majority of the factors previously 
discussed were derived from actual case 
studies of thick-seam operations and from 
research conducted on western thick 



16 



TABLE 3. - Importance of geologic factors for four thick-seam 
mining methods 



Geologic factor 



Cleats 

Joints and/or fractures.. 

Coal quality 

Petrography 

Lithology of roof rock... 
Lithology of floor rock.. 
Thickness and 3-D 

configuration 

Continuity 

Rolls 

Spontaneous combustion. . . 

Gas 

Water 

C Critcal to development 



Multislice 


Longwall 


Single 


Hydraulic 




caving 


pass 


mining 


X 


C 


X 


c 


X 


X 


X 


X 


X 


C 


X 


X 


- 


X 


- 


X 


X 


X 


X 


X 


X 


X 


X 


X 


c 


X 


c 


- 


c 


X 


X 


X 


c 


X 


X 


- 


X 


C 


X 


c 


X 


X 


X 


X 


X 


C 


X 


X 



X Important. 



deposits. Table 3 summarizes the fac- 
tors and their hypothesized importance to 
the different mining methods. 

All the factors listed will affect 
development in thick seams differently, 
depending on the geology of the site and 
the mining method. In high-face single- 
pass longwall, the thickness and change 
in thickness across the panel are impor- 
tant. Anything that thins the coal or 
displaces the seam, such as faulting 
or washouts, can have a deleterious 
effect on the face. Some features cannot 
easily be picked out prior to actual 
development. 

In multislice mining, thickness, change 
in thickness, and continuity of both 
seams are important. Another determinant 
of multislice use is coal quality, as 
zones of low quality in the coal may 
be preferentially left as web between 
the upper and lower slice. Rolls were 
previously mentioned. Also, when the 
panels are being developed, heating in 
the remaining coal may cause problems. 

Many factors are important in longwall 
caving. Cleats are hypothesized to be a 
critical factor in the caving operation 
in that the cleat development, density of 
major cleat planes cutting through the 
coal to be caved, and relative smoothness 
of the cleat surfaces help determine 



cavability as well as the block size of 
the caved material. Another important 
characteristic is the degree of fractur- 
ing in the seam to be mined; a highly 
fractured coal will cave more easily. 
Fractures will also help control block 
size. Coal quality is important from the 
marketing standpoint. Also, caving in a 
section of the coal that is high in ash 
(bone) may vary owing to both changes in 
the strength of the material and fracture 
and cleats within the section. Spontane- 
ous combustion is a critical considera- 
tion, depending on the amount of coal 
remaining in the gob after caving is 
completed. 

In hydraulic mining, cleat development 
helps determine the ease with which coal 
can be cut. As in longwall caving, spon- 
taneous ignition of coal remaining in the 
gob is also an important consideration. 

Hydraulic mining is viable because of 
its flexibility. The method can handle 
changes in thickness and has worked well 
in highly faulted discontinuous seams. 
Water, which is often a hazard in other 
operations, is often advantageous in 
hydraulic mining. Water that is derived 
from the surrounding strata can be added 
to the water used in mining, replenishing 
water lost from the supply due to evapo- 
ration and the raining process. 



17 



CONCLUSIONS 



Thick coal is common in many of the 
coal regions in the Western United 
States. In Colorado alone, 211 separate 
occurrences of thick, coal or seams split 
by less than 30 ft of interburden were 
found in 4 of the 8 coal regions. A sub- 
stantial number of occurrences are also 
evident in Utah and Wyoming. 

Ongoing Bureau of Mines research indi- 
cates that four mining methods — high-face 
single-pass longwall, multislice long- 
wall, longwall caving, and hydraulic 
mining — can be used to effectively ex- 
tract these seams. It is essential to 
the success of these methods that, on 
each site, a thorough geologic evaluation 
be performed. The resulting data are 
critical in determining the selection of 



the mining method and predicting problems 
that will be encountered in the use of 
the method. The principal factors that 
will affect the different mining methods 
follow: high-face single-pass longwall - 
thickness and changes in thickness across 
the panel; multislice longwall - thick- 
ness and changes in thickness and the 
presence of faults, washouts, and rolls; 
longwall caving - cleat development, coal 
quality, fracturing, and the tendency for 
spontaneous ignition; hydraulic raining - 
thickness, cleat development, and tend- 
ency for spontaneous ignition. All other 
factors listed in this report can also 
cause problems and must be considered in 
the initial mine assessment. 



REFERENCES 



1. Matson, T. K. , and D. H. White, Jr. 
The Reserve Base of Coal for Underground 
Mining in the Western United States. 
BuMines IC 8678, 1975, 238 pp. 

2. Trumbull, J., and F. F. Barnes. 
Coal Fields of the United States. U.S. 
Geol. Surv. Map, 1960, 2 maps, scale 
1:5,000,000. 

3. Rushworth, P., B. S. Kelso, and 
L. R. Ladwig. Map, Directory, and Sta- 
tistics of Permitted Colorado Coal Mines, 
1983. CO Geol. Surv. Map Ser. 23, 1983, 
128 pp. 

4. Nielsen, G. F. (ed). 1985 Keystone 
Coal Industry Manual. McGraw-Hill Mining 
Publications, 1985, pp. 870-1256. 

5. Oitto, R. H. Three Potential Long- 
wall Mining Methods for Thick Coal Seams 
in the Western United States. BuMines 
IC 8792, 1979, 34 pp. 

6. Kaiser Resources Ltd. (Canada). 
Hydraulic Mining. Undated, 7 pp. 

7. McCulloch, C. M. The Role of the 
Geologist in Coal Mining. Paper in the 
Proceedings of the Second Symposium on 
the Geology of Rocky Mountain Coal 
(Golden, CO, May 9-10, 1977). CO Geol. 
Surv. Res. Ser. 4, 1977, pp. 101-127. 



8. Ting, F. T. C. Coal Macerals. 
Ch. in Coal Structure, ed. by R. A. 
Meyers. Academic, 1982, pp. 7-49. 

9. Kent, B. H. , and H. H. Arndt. 
Geology of the Thompson Creek Coal Mining 
Area, Pitkin County, Colorado, as Related 
to Subsurface Hydraulic Mining Potential. 
U.S. Geol. Surv. Open File Rep. 80-507, 
1980, 81 pp. 

10. Vaninetti, G. E., K. D. Gurr, and 
R. S. Dewey. Effect of Geologic Features 
on Underground Coal Mine Productivity. 
Paper in the Proceedings of the Fifth 
Symposium on the Geology of Rocky Moun- 
tain Coal — 1982 (Salt Lake City, UT, May 
12-13, 1982). UT Geol. and Min. Surv. 
Bull. 118, 1982, pp. 129-142. 

11. Bottril, F., S. Lewis, and L. R. 
Stace. Strata Control in Thick Seams in 
the United Kingdom. Paper in Proceedings 
of the International Symposium on Thick 
Seam Mining (Dhanbad, India, May 4-6, 
1977). Indian School of Mines, 1977, 
7 pp. 

12. Boreck, D. L. , and J. N. Weaver. 
Coal Bed Methane Study of the 'Anderson 1 
Coal Deposit, Johnson County, Wyoming — A 



958? 310 



18 



Preliminary Report, U.S. Geol. Surv. Open 
File Rep. 84-831, 1984, 16 pp. 

13. Jeremic, M. L. Elements of Hy- 
draulic Coal Mine Design. Gulf Publ. 
Co., Houston, TX, 1983, 158 pp. 

14. Boreck, D. L. , and D. K. Murray. 
Colorado Coal Reserve Depletion Data and 
Coal Mine Summaries. CO Geol. Surv. 
Open File Rep. 79-1, 1979, 65 pp. 

15. Bacharach, J. P. L. , E. A. C. 
Chamberlain, D. A. Hall, S. B. Lord, and 
D. J. Steele. A Review of Spontaneous 
Combustion Problems and Controls With 
Application to U.S. Coal Mines. U.S. 
Dep. Energy Rep. T10-28879, Sept. 1978, 
127 pp. 

16. Ahcan, R. Results of Trials With 
Two-Level Longwall Face Mining and Stor- 
ing in the Velenje Lignite Mine. 
Rudarsko-Metalurski Zbornik, No. 2, 1969, 
pp. 147-164 (Engl, transl.). 

17. Stach, E., M. Mackowsky, M. Teich- 
muller, G. H. Taylor, D. Chandra, and 
R. Teichmuller. Stach 's Textbook of Coal 
Petrology. Gebruder Borntraeger, Berlin, 
1975, 428 pp. 

18. Ghose, A. K. Underground Methods 
of Extraction of Thick Coal Seams - A 
Global Survey. Min. Sci. and Technol. Q. 
v. 2, No. 1, 1984, pp. 17-32. 

19. Khanna, R. K. A Proposed Method 
for Extraction of Developed Pillars With 
Caving in Argada Seam. Paper in Pro- 
ceedings of the International Symposium 
on Thick Seam Mining (Dhanbad, India, 
May 4-6, 1977). Indian School of Mines, 
1977, 6 pp. 

20. Moebs, N. N. , and R. M. Stateham. 
Geologic Factors in Coal Mine Roof 
Stability - A Progress Report. BuMines 
IC 8976, 1984, 27 pp. 

21. Ahcan, R. , S. Janezic, I. Berger, 
M. Kresic, and B. Djukic. Determination 
of Guidelines for Mining Thick Coal Seams 
According to the Velenje Mining Method in 
the Collieries of the SFR Yugoslavia. 
Paper in 12th World Mining Conference 
(New Delhi, India, Nov. 19-23, 1984). 
World Mining Conference, Stockholm, Swe- 
den, 1984, 18 pp. 

22. Chugh, Y. P., R. D. Caudle, and 
V. K. Agarwala. Premining Investiga- 
tions for Longwall Coal Mining. Ch. in 
State-of-Art of Ground Control in Long- 
wall Coal Mining and Mine Subsidence, ed. 



by Y. Chugh and M. Karmis. Soc. Min. 
Eng. AIME, 1982, pp. 3-12. 

23. Ryer, T. A. Deltaic Coals of Fer- 
ron Sandstone Member of Mancos Shale; 
Predictive Model for Cretaceous Coal- 
Bearing Strata of the Western Interior. 
Am. Assoc. Pet. Geol. Bull., v. 65, No. 
11, 1981, pp. 2323-2340. 

24. Law, B. E. Large-Scale Compaction 
Structures in the Coal Bearing Fort Union 
and Wasatch Formations, Northeast Powder 
River Basin, Wyoming. Paper in Proceed- 
ings of the 25th Annual Field Confer- 
ence Guidebook-Geology and Energy Re- 
sources of the Powder River (Casper, 
WY, Sept. 1976). WY Geol. Assoc, 1976, 
pp. 221-229. 

25. Dunrud, C. R. Coal Mine Subsid- 
ence and Fires in the Sheridan, Wyoming 
Area. Paper in Proceedings of the Fourth 
Symposium on the Geology of Rocky Moun- 
tain Coal - 1980 (Golden, CO, Apr. 28-29, 
1980). CO Geol. Surv. Resour. Ser. 10, 
1980, pp. 26-34. 

26. Hobbs, R. G. Methane Occurrences, 
Hazards, and Potential Resources, Recluse 
Geologic Analysis Area, Northern Campbell 
County, Wyoming. U.S. Geol. Surv. Open 
File Rep. 78-401, 1978, 20 pp. 

27. Bise, C. J. , and R. V. Ramani. 
Equipment, Ground Controls, and Safety 
Considerations for Thick-Seam Underground 
Coal Mining. Soc. Min. Eng. AIME pre- 
print 81-326, 1981, pp. 4-5. 

28. Diamond, W. P., J. C. LaScola, and 
D. M. Hyman. Results of Direct-Method 
Determination of the Gas Content of U.S. 
Coalbeds. BuMines IC 9067, 1986, 95 pp. 

29. Van Voast, W. A., R. B. Hedges, 
and J. J. McDermott. Hydrologic Charac- 
teristics of Coal-Mine Spoils, Southeast- 
ern Montana. MT Univ. Joint Water Res. 
Center Rep. 94, June 1978, 34 pp. 

30. Ahcan, R. Precaution Measures 
Against Sudden Inrushes of Water and Mud 
in Collieries of SFR Yugoslavia. Paper 
in World Congress of Water in Mining 
and Underground Work (Granada, Spain, 
Sept. 18-22, 1978). Asociacion Nacional 
de Ingenierof de Minaf, Madrid, 1978, 
pp. 585-601 (Engl, transl.). 

31. Strong, W. J. Water - A Necessity 
of Life? Min. Eng. (London), v. 44, No. 
276, Sept. 1984, pp. 159-165. 



4 U.S. GOVERNMENT PRINTING OFFICE: 1986—605-017/40,096 



INT.-BU.OF MINES, PGH., PA. 28373 



U.S. Department of the Interior 
Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh, Pa. 15236 



AN EQUAL OPP- 



OFFIC1AL BUSINESS 
PENALTY FOR PRIVATE USE. MOO 

] Do not wish to receive this 
material, please remove 
from your mailing list. 

] Address change. Please 
correct as indicated* 



)RTUNITY EMflLOY^ 

TAKE PRIDE 
JN AMERICA / 



LIBRARY OF CONGRESS RCM-24 

CATALOGING IN PUBLICATIONS DIV 
WASHINGTON DC 20540 



us. official; 

U.S PI 










» ^ 



^ * » _ a A*" 





» >»» •"» 



o V 



- W 

















* .... °«fe.^ 



* -r^v -v*^- 















C° .<^L>.. °o 



» • «-> "«* 



V'^'V V^-'/ v : «^\«* °o.*: 




^3 V 



9 \»L^L'« 











t/ \wc??-/ %•#/ \#/ \W/\ 

te-- % /••*•% /«-> /••«;\ **w. 






^°- 






"by 

* ^ v 






»b/ ' 







• "^S A^ » 












"OK 


























• ""W* : -^£: %^' : -aK: \/ .-i$S&-.'^ 






^^ ° 












•^-^- t4 * 



^.•^fc.V ./,-^-.^ >V-3^1,^ ^■■^•■^ ^.-^:^ 



